CA Seismic flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Base shear formula

The total design lateral force V = Cs·W, where Cs is the seismic response coefficient and W is the effective seismic weight of the structure including applicable portions of other loads.

Seismic response coefficient Cs

Cs = SDS/(R/Ie) for short-period structures, capped by SD1/(T·(R/Ie)), and subject to minimum values of 0.044·SDS·Ie and 0.01.

Response modification factor R

A system-dependent factor that reduces elastic seismic demand to account for ductility and overstrength; higher R values indicate more energy-dissipating structural systems.

Importance factor Ie

A multiplier on seismic demand reflecting occupancy risk category; values range from 1.0 for Risk Category I–II to 1.5 for Risk Category IV essential facilities.

Site class A through F

A classification of subsurface soil and rock based on shear wave velocity, SPT blow count, or undrained shear strength; softer sites (E, F) amplify long-period ground motion.

Site coefficient Fa

A short-period amplification factor applied to the mapped short-period spectral acceleration Ss to obtain SMS; it increases as site stiffness decreases and as Ss decreases.

Site coefficient Fv

A 1-second-period amplification factor applied to the mapped 1-second spectral acceleration S1 to obtain SM1; it is largest for soft soils and low S1 values.

Design spectral acceleration SDS

Two-thirds of the site-adjusted short-period spectral acceleration SMS; it anchors the constant-acceleration portion of the design response spectrum.

Design spectral acceleration SD1

Two-thirds of the site-adjusted 1-second spectral acceleration SM1; it defines the descending branch of the design response spectrum for periods beyond Ts = SD1/SDS.

Approximate period Ta

Ta = Ct·hn^x, an empirical estimate of fundamental period based on height and structural system; the computed period used in analysis may not exceed Cu·Ta.

Period coefficient Ct and x

Empirical constants in Ta = Ct·hn^x: 0.028/0.8 for steel moment frames, 0.016/0.9 for concrete moment frames, and 0.020/0.75 for other systems.

Vertical force distribution Fx

Lateral force at level x given by Fx = Cvx·V, with Cvx proportional to wx·hx^k normalized by the sum over all levels; k varies from 1 to 2 with period.

Exponent k in distribution

A period-dependent exponent in the vertical force distribution: k = 1.0 for T ≤ 0.5 s, k = 2.0 for T ≥ 2.5 s, and linear interpolation between those limits.

Story drift Δ

The design drift equals Cd·δxe/Ie, where δxe is the elastic deflection from analysis; it must not exceed the code allowable drift ratio for the risk category.

Allowable drift ratios

Maximum permitted drift ratios of approximately 0.020 for Risk Categories I–II, 0.015 for III, and 0.010 for IV, applied to the amplified design drift.

Deflection amplification Cd

A system-dependent factor that amplifies elastic analysis deflections to estimate inelastic drift; it is typically larger than R for the same structural system.

Redundancy factor ρ

A factor of 1.0 or 1.3 applied to seismic base shear for structures in SDC D–F, depending on whether the lateral system meets redundancy conditions.

Overturning moment

The sum of design lateral forces above a level multiplied by their heights above that level; it produces axial force couples in vertical elements and must be resisted by the foundation.

Stability coefficient θ

θ = Px·Δ/(Vx·hsx·Cd); when θ exceeds 0.10, second-order effects must be considered, and θ may not exceed θmax = 0.5/(β·Cd).

P-delta effect

Additional internal forces and drift caused by gravity loads acting on the laterally displaced structure; significant when the stability coefficient θ exceeds 0.10.

Seismic Design Category

A classification from A to F based on risk category and design spectral accelerations SDS and SD1; it determines detailing, analysis, and system restrictions.

Diaphragm flexibility

A floor or roof system may be idealized as rigid, semi-rigid, or flexible; flexible diaphragms distribute lateral force to vertical elements in proportion to tributary area.

Accidental torsion

A design eccentricity equal to 5% of the dimension perpendicular to the applied force, plus inherent torsional irregularity effects, applied to account for unanticipated mass and stiffness offsets.

Torsional amplification Ax

A factor applied to accidental torsion at a level where the maximum drift exceeds 1.2 times the average drift; Ax = (δmax/(1.2·δavg))^2, capped at 3.0.

Structural irregularity

A configuration condition such as vertical mass, stiffness, or geometric discontinuity, or plan torsion or reentrant corner, that triggers stricter analysis and design requirements.

Equivalent lateral force procedure

A static analysis method applying base shear distributed over the height; permitted for regular structures within height and period limits, otherwise modal response spectrum analysis is required.

Modal response spectrum analysis

A dynamic analysis combining peak responses of significant modes using modal combination rules such as SRSS or CQC, required for tall, irregular, or long-period structures.

Scaling of design forces

Response spectrum analysis base shear must be scaled to at least 85% of the equivalent lateral force base shear when the ELF procedure is also permitted for the structure.

Seismic weight W

Total dead load plus specified portions of live load, partition weight, and snow; it excludes most transient live loads but includes permanent equipment and stored material allowances.

Load combination with seismic

Gravity-plus-seismic combinations such as 1.2D + Ev + Eh + L + 0.2S, where Ev is 0.2·SDS·D and Eh is the horizontal seismic effect including ρ.

Soil amplification

The increase in ground surface shaking relative to rock motion caused by soft soil layers; it is strongest at periods near the site period and is captured by Fa and Fv.

Near-fault effects

Ground motions close to a fault can produce large long-period velocity pulses; design spectral values at long periods may be amplified by near-fault factors.